What ESAPI is? The ESAPI is a free and open collection of all the security methods that a developer needs to build a secure web application. You can just use the interfaces and build your own implementation using your company's infrastructure. Or, you can use the reference implementation as a starting point. In concept, the API is language independent. However, the first deliverables from the project are a Java API and a Java reference implementation. Efforts to build ESAPI in .NET and PHP are already underway.